Fiber optic amplitude sensing, where the measurement is determined by the intensity of the returned light beam in an optical fiber, has been in use for some time; however, connector and bending losses, particularly in harsh environs, make such sensing inaccurate. Another technique in use is phase modulation, but this too is sensitive to the environment and is costly. This application describes a technique for the modulation of an optical beam by wavelength or color; specifically, the optical sensor described is a filter whose spectral band edge is a function of the parameter to be measured. This general class of optical sensors was first established by Christensen.sup.1 with a transmission/absorption spectral filter and band edge in GaAs designed to measure temperature. Quick, James and Coker.sup.2 developed a spectral line resonance filter using a broad-band source, optical fibers and a Fabry-Perot interferometric cavity to measure temperature; Lewis.sup.3 at Litton Poly-Scientific used a broad-band source, optical fibers, and color filters to determine rotary position. Also, Knox, Marshall and Murray.sup.4 used a broad-band source, a monochromator, and birefringent filter to measure temperature; Jones and Spooncer.sup.5 used a broad-band source, a monochromator, and birefringent filter to measure pressure. Finally, Seaver.sup.6,7,8 established the critical reflection/transmission filter and band edge technique to measure the index of refraction (1987) and temperature (1986).
When an optical beam is incidence upon a surface at or greater than the critical angle, it is totally reflected. At angles less than critical, it is little reflected, and the transition between these two conditions is very abrupt. This is the principle of critical angle optical sensing. Historically, this has been used by measuring the angle of incidence of a single, or narrow, wavelength beam at critical reflection. This became the Abbe critical angle refractometer.sup.9 for measuring the index of refraction of a test fluid. More recently, Seaver.sup.6 held the angle constant and measured the wavelength at critical reflection. This is the critical wavelength refractometer for measuring the index of refraction of a test material.